1. Field of the Invention
The present invention relates generally to a switching apparatus for PCM (pulse code modulated) or video signals and is directed more particularly to a switching apparatus for PCM or video signals suitable for use with a PCM recording and reproducing apparatus which utilizes a VTR (video tape recorder) and so on.
2. Description of the Prior Art
With a prior art analog tape deck, the variation of sound magnitude is recorded after being converted to the magnetic force of the tape and the variation of sound in time is recorded on a tape in its travelling direction. Therefore, the properties of a head and the tape affect on the dynamic range and distortion factor of the sound, while the property of the tape transporting system affects on the sound pitch, i.e. wow and flutter, speed deviation and modulation noise. According to the PCM recording method, the variation of sound in time is once standardized, then the amplitude thereof is quantized and recorded on a tape. Therefore, according to this method, if "0" or "1" can be discriminated upon reproduction, the original waveform can be completely reproduced. By the digitization of such the signal, obtained are advantages that there are no speed deviation, no wow and flutter and no level fluctuation and so on. However, due to the fact that the frequency of a signal to be processed is high it is rather difficult that the advantage of the PCM record and reproduction is sufficiently realized in a tape recorder in which a tape is merely transported for a fixed head.
Therefore, it is known in the art that a PCM signal is recorded and reproduced by a VTR (video tape recorder) in which a tape is transported at a high speed while a head is rotated. As well known, in the VTR a video signal is frequency-modulated, horizontal and vertical synchronizing signals are added in consideration of the tape speed, rotation speed of a rotary head and so on and then, the signals are recorded on a tape.
When an audio PCM signal is recorded and reproduced by a VTR, which is used to record and reproduce a standard television signal, for example, NTSC television signal, since the format of a PCM signal must follow the standard television signal, in one field there are contained 245H for a data block, 1H for a control signal to be pliable for various applications, and horizontal and vertical synchronizing signals same as those of the standard television signal. Further, in one frame there is contained such a portion where no signal is recorded for the head switching at every vertical synchronization. Thus, a signal format different from the signal process of a video signal is used due to the necessity to time-compress a continuous input audio signal and so on.
Therefore, in a PCM recording and reproducing machine using a VTR and so on, since the suitable signal process is different between the video signal reproduction and PCM signal reproduction, the signal process must be switched, if necessary. In general, upon video signal reproduction mode, in order to decrease the drop-out noise due to a tape and so on, a so-called drop-out compensating circuit is used in which a 1H (H is the horizontal period) delay line is employed so as to use an FM signal before 1H by utilizing the line correlation of a television picture or a so-called H correlation noise reduction circuit is used to improve an S/N ratio which utilizes such fact that the line correlation of the reproduced luminance signal to a 1H delay signal is strong and a differential output thereof contains noise components with no line correlation. These circuits mentioned just above are preferable when the video signal is reproduced but is not preferred when the PCM signal is reproduced since if the digitized PCM signal passes through these circuit, its data format is deformed. Therefore, upon PCM signal reproduction mode it is necessary to change over the drop-out compensation circuit or H correlation noise reduction circuit such that the PCM signal does not pass through the above circuit.
In the art, the above change-over is carried out manually, so that the handling is rather troublesome.